Studies in our laboratory suggest that, in mammalian cells, the synthesis of transfer RNA may be regulated by deacylated tRNA. Recently it was suggested by Grummt et al. (Cell 7: 439, 1976) that ribosomal RNA synthesis may be regulated by deacylated tRNA via an effect on the GTP pool size. We propose to examine whether the regulation of tRNA synthesis proceeds by a similar mechanism. We also plan to study the effect of deacylated tRNA on the synthesis and turnover of specific tRNAs. Such studies may provide an explanation for the observation that the tRNA composition of differentiated cells correlates with the amino acid composition of the protein being synthesized.